Four-wheel drive vehicle

ABSTRACT

A rear differential in a four-wheel drive vehicle includes a left and right clutches adapted to distribute a driving force transmitted from front wheels through a propeller shaft to left and right rear wheels. If the engagement of the left and right clutches is released, only the front wheels are driven and in this manner, the vehicle is brought into a front wheel-drive state. If the left and right clutches are brought into their engaged states, both of the front wheels and the rear wheels are driven and in this manner, the vehicle is brought into a four-wheel drive state. By changing the engagement forces of the left and right clutches, different driving forces can be distributed to the left and right rear wheels. In addition, when a driver operates a differential lock switch, the left and right clutches, are brought into their engaged states with the maximum engagement force, thereby integrally coupling the propeller shaft to the left and right rear wheels to provide a differential-locked state. Thus, a differential lock mechanism can be provided to the differential in a simple structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a four-wheel drive vehicle which iscapable of being switched over between a two-wheel drive state and afour-wheel drive state.

2. Description of the Related Art

The main type of the current four-wheel drive vehicles is a full-timetype four-wheel drive vehicle including a front differential adapted todistribute a driving force between left and right front wheels, a centerdifferential adapted to distribute the driving force between the frontwheels and rear wheels, and a rear differential adapted to distributethe driving force between the left and right rear wheels. A part-timetype four-wheel drive vehicle includes a clutch for intermittentlytransmitting the driving force to a transfer means adapted to transmitthe driving force for the front wheels to the rear wheels.

To transmit the driving force to the left and right rear wheels, forexample, when the vehicle runs out of a muddy place, the full-time typefour-wheel drive vehicle is accompanied by a problem that it isnecessary to mount a differential lock device on the center differentialand the rear differential, resulting in an undesirabe increase in thenumber of parts and a complicated structure. The part-time typefour-wheel drive vehicle is accompanied by a problem that becauseclutches are provided on the transfer means, the number of parts isincreased, and the structure is complicated.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apart-time type four-wheel drive vehicle which is capable of beingswitched over between a two-wheel drive state and a four-wheel drivestate, wherein a differential lock mechanism is provided to thedifferential in a simple structure.

To achieve the above object, according to the present invention, thereis provided a four-wheel drive vehicle designed so that a driving forcefor one set of a set of front wheels and a set of rear wheels connectedto an engine is distributed to the other set of the wheels through aninput shaft and a differential, wherein the differential includes leftand right clutches which are capable of coupling the other left andright wheels to the input shaft with any engagement force, and theclutches are brought into their engaged states with the maximumengagement force in response to the operation of an input means providedby a driver.

With such an arrangement, if the engagement of the left and rightclutches mounted on the differential is released, one set of the frontwheels and the rear wheels is driven, and in this manner, the vehiclecan be brought into a two-wheel drive state. If the left and rightclutches are brought into their engaged states, both of the front andrear wheels are driven, and in this manner, the vehicle can be broughtinto a four-wheel drive state. In this case, by changing the engagementforces of the left and right clutches, different driving forces candistributed to the other left and right wheels, and in this manner, adifferential-lock mechanism can be exhibited. Moreover, when the vehicledriver operates the input means, the left and right clutches are broughtinto their engaged states with the maximum engagement force, and theinput shaft and the other left and right wheels are integrally coupledto each other to provide a differential-locked state. For this purpose,only the engagement forces of the left and right clutches are controlledand hence, the structure is extremely simple.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 10 illustrate an embodiment of the present invention, wherein

FIG. 1 is an illustration of the entire arrangement of a four-wheeldrive vehicle;

FIG. 2 is a plan view of the entire rear differential;

FIG. 3 is an enlarged view of a portion of the differential shown inFIG. 2;

FIG. 4 is an enlarged view of a portion of the differential shown inFIG. 2;

FIG. 5 is an enlarged view of a portion of the differential shown inFIG. 2;

FIG. 6 is an enlarged sectional view taken along a line 6--6 in FIG. 4;

FIG. 7 is an enlarged view of an essential portion shown in FIG. 4;

FIG. 8 is a sectional view taken along a line 8--8 in FIG. 3;

FIG. 9 is a sectional view taken along a line 9--9 in FIG. 8; and

FIG. 10 is a graph illustrating the relationship between the vehiclespeed and the maximum transmitted torque of the clutch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described by way of an embodiment withreference to the accompanying drawings. Referring to FIG. 1, afour-wheel drive vehicle V includes an engine E laterally mounted at afront portion of a vehicle body, a transmission M provided integrallywith the engine E, a front differential D_(F) which connects thetransmission M to drive shafts 1_(L) and 1_(R) of left and right frontwheels W_(FL) and W_(FR), a transfer T which connects the frontdifferential D_(F) to a propeller shaft 2, and a rear differential D_(R)which connects the propeller shaft 2 to drive shafts 3_(L) and 3_(R) ofleft and right rear wheels W_(RL) and W_(RR). The rear differentialD_(R) is capable of controlling the transmitting of a driving force tothe drive shafts 3_(L) and 3_(R) of the rear wheels W_(RL) and W_(RR).When the transmitting of the driving force is cut off, the vehicle isbrought into a front wheel drive state in which only the front wheelsW_(FL) and W_(FR) are driven, and when the driving force is transmitted,the vehicle is brought into a four-wheel drive state in which both ofthe front wheels W_(FL) and W_(FR) and the rear wheels W_(RL) and W_(RR)are driven. Further, in the four-wheel drive state, the reardifferential D_(R) is capable of controlling the distribution of thedriving force to the left and right rear wheels W_(RL) and W_(RR) to anyextent.

Connected to an electronic control unit U are a front wheel speed sensorS₁ for detecting a front wheel speed based on a number of rotations ofthe propeller shaft 2, a pair of rear wheel speed sensors S₂, S₂ fordetecting rear wheel speeds based on numbers of rotations of the leftand right drive shafts 3_(L) and 3_(R) of the rear wheels W_(RL) andW_(RR), a steering angle sensor S₃ for detecting a steering angle of asteering wheel 4, a yaw rate sensor S₄ for detecting a yaw rate of thevehicle body, a lateral acceleration sensor S₅ for detecting a lateralacceleration of the vehicle body, and a differential lock switch S₆ forlocking the rear differential D_(R). The electronic control unit Ucontrols left and right electromagnetic clutches C_(L) and C_(R) (whichwill be described hereinafter) mounted in the rear differential D_(R)based on signals from the sensors S₁ to S₅ and the differential lockswitch S₆.

The structure of the rear differential D_(R) will be described withreference to FIGS. 2 to 9. The rear differential D_(R) has asubstantially laterally symmetric structure and hence, with the regardto the lateral symmetric portions, one namely, the left portion of theleft and right portions will be described, and a duplicate descriptionof the right portion is omitted.

The rear differential D_(R) includes a casing means which is dividedinto a front center casing 11, a rear center casing 12 coupled to a rearsurface of the front center casing 11 by a plurality of bolts 14 (seeFIG. 8), a left side casing 13_(L) coupled to left sides of the centercasings 11 and 12 by a plurality of bolts 15, and a right side casing13_(R) coupled to right sides of the center casings 11 and 12 by aplurality of bolts 15.

An input shaft 18 is supported in the front center casing 11 by a pairof tapered roller bearings 16 and 17, and coupled at its front end to arear end of the propeller shaft 2 (see FIG. 1) through a coupling 19.The front wheel speed sensor S₁ opposed to a rotor 20 fixed to the inputshaft 18 to detect a number of rotations of the input shaft 18 is fixedto the front center casing 11 by a bolt 21. A hollow clutch drive shaft23 is supported at its opposite ends in the front center casing 11 andthe rear center casing 12 through a pair of ball bearings 22, 22, and adriven bevel gear 26 integrally formed at a rear end of the input shaft18 is meshed with a follower bevel gear 25 fixed to the clutch driveshaft 23 by a bolt 24. The input shaft 18 and the clutch drive shaft 23are in offset locations and are not on the same plane. Therefore, thefollower bevel gear 25 and driven bevel gear 26, which are of a hypoidtype, are used.

A left output shaft 29_(L) is supported coaxially with the clutch driveshaft 23 by a ball bearing 27 mounted on the left side casing 13_(L) anda needle bearing 28 mounted at a left end of the clutch drive shaft 23.The left drive shaft 3_(L) (see FIG. 1) is coupled at its right end to aleft end of the left output shaft 29_(L) protruding from the left sidecasing 13_(L) through a coupling 30. The rear wheel speed sensor S₂opposed to a rotor 31 fixed to the left output shaft 29_(L) to detect anumber of rotations of the left output shaft 29_(L) is fixed to the leftside casing 13_(L) by a bolt 32.

The left electromagnetic clutch C_(L) accommodated in the left sidecasing 13_(L) includes a clutch outer 36 spline-coupled to the left endof the clutch drive shaft 23, a clutch inner 37 spline-coupled to aright end of the left output shaft 29_(L), a plurality of clutch disks38 axially slidably but non-rotatably carried on an inner periphery ofthe clutch outer 36, a plurality of clutch plates 39 axially slidablybut non-rotatably carried on an outer periphery of the clutch inner 37and superposed alternately on the clutch disks 38, and a clutch piston40 axially slidably carried on the outer periphery of the clutch inner37 for bringing the clutch disks 38 and the clutch plates 39 into closecontact with each other.

A ball cam mechanism 44 is provided on the outer periphery of the leftoutput shaft 29_(L) and is comprised of a stationary cam member 41, amovable cam member 42 and a plurality of balls 43. A left side of thestationary cam member 41 is opposed to a right side of the ball bearing27 with a thrust bearing 45 interposed therebetween, and a right side ofthe movable cam member 42 is opposed to a left side of the clutch inner37 with a spring 46 interposed therebetween and opposed to a left sideof the clutch piston 40 with a small gap left therebetween. An outerperipheral surface of the stationary cam member 41 is spline-coupled at48 to an inner peripheral surface of a coil housing 47 which will bedescribed hereinafter, and an inner peripheral surface of the movablecam member 42 is spline-coupled at 49 to an outer peripheral surface ofthe left output shaft 29_(L).

As can be seen by reference to FIG. 6 together with other Figures,triangular cam grooves 41₁ and 42₁ are defined at predetermineddistances in opposed surfaces of the cam members 41 and 42 of the ballcam mechanism 44, and the balls 43 are disposed between opposed camgrooves 41₁ and 42₁.

As can be seen from FIG. 7, a solenoid 50 is disposed radially outsidethe ball cam mechanism 44 and includes an annular coil 52 covered withan insulating material 51, the annular coil housing 47 which covers aninner peripheral surface, an outer peripheral surface and a right sideof the coil 52, and an annular armature 54 disposed on a right side ofthe coil housing 47. The coil 52 is fixed to the left side casing 13_(L)by a means which is not shown, and the coil housing 47 is supported forrotation about the left output shaft 29_(L) through the ball cammechanism 44. An outer periphery of the armature 54 is spline-coupled at55 to the clutch outer 36, and a right side of the armature 54 isopposed to the left side of the clutch piston 40 with a bellevillespring 56 interposed therebetween.

A left end (an end opposite from the armature 54) of the coil housing 47protrudes leftwards by a distance L from a left end of the coil 52,whereby a magnetic closed circuit shown by a bold line is easily formedto enhance the magnetic flux density to increase the attracting force ofthe armature 54, as compared with a case where the left end of the coilhousing 47 is terminated at a location on the right of the left end ofthe coil 52. Gaps α, α are defined between the coil 52 fixed to the leftside casing 13_(L) and the coil housing 47 which is rotated relative tothe coil 52, but by minimizing the size of the gaps α, α the magneticflux density can be further enhanced. Further, by forming the coil 52,the coil housing 47 and the armature 54 from a material having a highrelative magnetic permeability such as silicon, a permalloy, and thelike, the magnetic closed circuit can be cut off in magnetism to preventthe magnetic flux from being leaked to another member.

A gap β is defined between an outer peripheral surface of the movablecam member 42 and the inner peripheral surface of the coil housing 47,and a gap γ is defined between an inner peripheral surface of thestationary cam member 41 and the outer peripheral surface of the leftoutput shaft 29_(L). These gaps β and γ enable the amount of magneticflux leaked from the magnetic closed circuit through the stationary andmovable cam members 41 and 42 to the left output shaft 29_(L) to besuppressed to the minimum, thereby increasing the attracting force ofthe armature 54 and reducing the electric power consumed by the coil 52.

As can be seen by reference to FIGS. 8 and 9, an oil pump 61accommodated in an internal space in the front center casing 11 and arear center casing 12 is comprised of a trochoid pump and includes apump housing 63 fixed to an inner surface of the front center casing 11by bolts 62, 62 a pump cover 65 coupled to the pump housing 63 by bolts64, an internally toothed outer rotor 66 rotatably accommodated withinthe pump housing 63 and the pump cover 65, and an externally toothedinner rotor 67 fixed to an outer periphery of the clutch drive shaft 23and meshed with the outer rotor 66.

A lubricating oil is stored in a space below the front and rear centercasings 11 and 12. An oil strainer 70 is mounted in an oil passage 69extending downwards from an intake port 68 defined below the pumphousing 63 and the pump cover 65, and is immersed in the oil. Adischarge port 71 is defined above the pump housing 63 and the pumpcover 65 to communicate with an oil passage 23₂ axially defined in theclutch drive shaft 23 through an oil bore 23₁ radially defined in theclutch drive shaft 23. The internal space in the front and rear centercasings 11 and 12 communicates with an internal space in the left andright side casings 13_(L) and 13_(R) through a plurality ofthrough-bores 11₁ and 12₁.

A right end of an oil passage 29₁ axially defined in the left outputshaft 29_(L) communicates with a left end of the oil passage 23₂ axiallydefined in the clutch drive shaft 23. Oil bores 29₂ and 29₃ radiallyextending from the oil passage 29₁ are defined in the left output shaft29_(L). One oil bores 29₂ face an oil bore 37₁ defined in the clutchinner 37, and the other bores 29₃ face the thrust bearing 45 disposedbetween the ball bearing 27 and the ball cam mechanism 44.

The operation of the embodiment of the present invention having theabove-described construction will be described below.

At the start of the vehicle, a driving force from the engine E is firsttransmitted to the left and right front wheels W_(FL) and W_(FR) throughthe transmission M, the front differential D_(F) and the drive shafts1_(L) and 1_(R). The driving force from the engine E is also transmittedto the rear differential D_(R) through the propeller shaft 2 to rotatethe input shaft 18, the driven bevel gear 26, the follower bevel gear 25and the clutch drive shaft 23. However, the left and rightelectromagnetic clutches C_(L) and C_(R) are in their non-engaged statesand hence, the rear wheels W_(RL) and W_(RR) are not driven. At thistime, the rotational speeds of the front wheels are detected by thefront wheel speed sensor S₁ mounted on the input shaft 18 of the reardifferential D_(R), and the rotational speeds of the rear wheels aredetected by the rear wheel speed sensors S₂, S₂ mounted on the left andright output shafts 29_(L) and 29_(R) of the rear differential D_(R).However, at a moment when the driving force has been transmitted to thefront wheels W_(FL) and W_(FR), the driving force is still nottransmitted to the rear wheels W_(RL) and W_(RR) due to the fact thatthe left and right electromagnetic clutches C_(L) and C_(R) are in theirnon-engaged states. Therefore, a differential rotation is producedbetween the front wheels W_(FL) and W_(FR) and the rear wheels W_(RL)and W_(RR). When the differential rotation between the front wheelsW_(FL) and W_(FR) and the rear wheels W_(RL) and W_(RR) is detected, theleft and right electromagnetic clutches C_(L) and C_(R) are brought intotheir engaged states based on a signal from the electronic control unitU, thereby permitting the rotation of the clutch drive shaft 23 to betransmitted to the rear wheels W_(RL) and W_(RR) through the left andright output shafts 29_(L) and 29_(R) and the left and right driveshafts 3_(L) and 3_(R). In this manner, the vehicle V is brought intothe four-wheel drive state.

The operation of the electromagnetic clutches C_(L) and C_(R) will bedescribed with the left electromagnetic clutch C_(L) shown in FIG. 4being taken as an example. When the solenoid 50 is in its non-energizedstate, the attraction of the armature 54 to the coil housing 47 has beenreleased, and hence, the coil housing 47 and the armature 54 rotaterelative to each other. In this state, the clutch drive shaft 23, theclutch outer 36, the clutch disks 38 and the armature 54 are in theirintegrated states, and the left output shaft 29_(L), the clutch inner37, the clutch piston 40, the ball cam mechanism 44 and the coil housing47 are also in their integrated states. Therefore. the transmission ofthe power from the clutch drive shaft 23 to the left output shaft 29_(L)has been cut off by slipping of the armature 54 relative to the coilhousing 47.

When the coil 52 of the solenoid 50 is energized by a command from theelectronic control unit U, the armature 54 is attracted to andintegrated with the coil housing 47. As a result, the rotation of theclutch drive shaft 23 is transmitted through the clutch outer 36, thearmature 54 and the coil housing 47 to the stationary cam member 41 ofthe ball cam mechanism 44, thereby producing the relative rotationsshown by arrows A and B in FIG. 6 between the stationary cam member 41integrated with the clutch drive shaft 23 and the movable cam member 42integrated with the output shaft 29_(L). When the stationary cam member41 and the movable cam member 42 have been rotated relative to eachother, the movable cam member 42 is moved rightwards away from thestationary cam member 41 against a biasing force of the spring 46 by areaction force received by the cam grooves 41₁ and 42₁ from the balls43, and pushes the clutch piston 40 rightwards to bring the clutch disks38 and the clutch plates 39 into engagement with each other.

Thus, the clutch outer 36 is coupled directly to the clutch inner 37through the clutch disks 38 and the clutch plates 39, and the leftelectromagnetic clutch C_(L) is brought into the engaged state, therebypermitting the rotation of the clutch drive shaft 23 to be transmittedto the left output shaft 29_(L). When the left and right electromagneticclutches C_(L) and C_(R) have been brought into the engaged states inthe above manner, the left and right rear wheels W_(RL) and W_(RR) aredriven. In this manner, the vehicle V is brought into the four-wheeldrive state.

The rear differential D_(R) is capable of generating a differencebetween the engagement forces of the left and right electromagneticclutches C_(L) and C_(R) by controlling a value of electric currentsupplied to the coils 52, 52 of the left and right solenoids 50, 50, sothat any torque is distributed to the left and right rear wheels W_(RL)and W_(RR), thereby controlling the steering characteristic of thevehicle. A reference yaw rate is calculated based on a steering angledetected by the steering angle sensor S₃, a vehicle speed calculatedbased on outputs from the front wheel speed sensor S₁ and the rear wheelspeed sensors S₂, S₂, and a lateral acceleration detected by the lateralacceleration sensor S₅, for example. during turning of the vehicle V.This reference yaw rate is compared with an actual yaw rate detected bythe yaw rate sensor S₄. If the vehicle is in an over-steering tendencyor an under-steering tendency as a result of the comparison, a controlfor eliminating the oversteering tendency or the under-steering tendencycan be performed.

Specifically, when the vehicle is in the over-steering tendency, a yawmoment causing the vehicle body to be turned outwards as viewed duringthe turning of the vehicle can be generated to eliminate theover-steering tendency by increasing the engagement force of theelectromagnetic clutch C_(L) or C_(R) that is on the inner side duringturning of the vehicle, and decreasing the engagement force of theelectromagnetic clutch C_(L) or C_(R) on the outer side during turningof the vehicle. When the vehicle is in the under-steering tendency, ayaw moment causing the vehicle body to be turned inwards as viewedduring turning of the vehicle can be generated to eliminate theunder-steering tendency by decreasing the engagement force of theelectromagnetic clutch C_(L) or C_(R) on the inner side during turningof the vehicle, and increasing the engagement force of theelectromagnetic clutch C_(L) or C_(R) on the outer side during turningof the vehicle.

When a driver has operated the differential lock switch S₆, the left andright electromagnetic clutches C_(L) and C_(R) are brought into theengaged states by the maximum transmitted torque. In this manner, thevehicle V is brought into the four-wheel drive state and adifferential-locked state in which the left and right rear wheels W_(RL)and W_(RR) have been integrally coupled to each other, which cancontribute to an increase in driving force when the vehicle runs out ofa muddy place.

In this way, the four-wheel drive state and the front wheel drive statecan be easily switched over from one to another in a simple structure inwhich the two electromagnetic clutches C_(L) and C_(R) are merelyprovided with the rear differential D_(R). Moreover, any driving forcecan be distributed to the left and right rear wheels W_(RL) and W_(RR),and a differential locking mechanism can be provided.

The appropriate magnitude of the maximum transmitted torque transmittedto the rear wheels W_(RL) and W_(RR) through the rear differential D_(R)is varied depending upon the friction coefficient of a road surface. Itis desirable that for a road surface of a smaller friction coefficient,the maximum transmitted torque is decreased, and for a road surface of alarger friction coefficient, the maximum transmitted torque isincreased. If the maximum transmitted torque is set according to thefollowing expression: weight of rear wheel axle x friction coefficientof road surface x radius of tire, a maximum transmitted torque suitablefor the friction coefficient of a road surface can be obtained.

If the maximum transmitted torque is set at a larger value such that itis suited for a road surface of a higher friction coefficient such as anasphalt covered road, the required capacity of the electromagneticclutches C_(L) and C_(R) is increased, whereby the size of theelectromagnetic clutches C_(L) and C_(R) is increased. However, amaximum transmitted torque suitable for any of various road surfaces ofdifferent friction coefficients can be obtained by limiting theengagement forces of the electromagnetic clutches C_(L) and C_(R) on aroad surface of a lower friction coefficient such as a snow-laden road.If the maximum transmitted torque is set at a smaller value such that itis suited for a road surface of a lower friction coefficient, the sizeof the electromagnetic clutches C_(L) and C_(R) can be reduced and inits turn, the size of the rear differential D_(R) can be reduced, but ona road surface of a higher friction coefficient, the maximum transmittedtorque may be insufficient in some cases.

By setting the maximum transmitted torque transmitted to the rear wheelsW_(RL) and W_(RR) through the rear differential D_(R), so that themaximum transmitted torque assumes a maximum value when the vehiclespeed is lower, and the maximum transmitted torque is decreased with anincrease in vehicle speed, as shown in FIG. 10, the sizes of theelectromagnetic clutches C_(L) and C_(R), the driven bevel gear 26 andthe follower bevel gear 25 can be reduced, and the durability thereofcan be enhanced. Namely, the horsepower of the engine E transmitted tothe rear wheels W_(RL) and W_(RR) through the rear differential D_(R) isproportional to the product of the maximum transmitted torque and thevehicle speed. However, if the maximum transmitted torque is decreasedwith the increase in vehicle speed, it is possible to prevent thehorsepower transmitted to the rear wheels W_(RL) and W_(RR) from beingincreased in accordance with the increase in vehicle speed. Thus, evenif the sizes of the electromagnetic clutches C_(L) and C_(R), the drivenbevel gear 26 and the follower bevel gear 25 are reduced, it is possibleto prevent a reduction in durability due to the transmission of a largerhorsepower when at a higher vehicle speed.

Now, when the clutch drive shaft 23 of the rear differential D_(R) isrotated, the inner rotor 67 and the outer rotor 66 of the oil pump 61accommodated in the front and rear center casings 11 and 12 are rotated,thereby causing the oil stored in the front and rear center casings 11and 12 to be drawn from the oil strainer 70 via the oil passage 69 intothe intake port 68 and supplied from the discharge port 71 via the oilbore 23₁ into the oil passage 23₂ defined in the clutch drive shaft 23.The oil flowing from the oil passage 23₂ in the clutch drive shaft 23into the oil passages 29₁, 29₁ in the left and right output shafts29_(L) and 29_(R) flows through the oil bores 29₂ and 29₃ extendingradially from the oil passages 29₁, 29₁ to the outside of the clutchdrive shaft 23. A portion of this oil is passed through the oil bores37₁ defined in the clutch inner 37 to lubricate the clutch disks 38 andthe clutch plates 39, and another portion of the oil lubricates the ballbearings 27, 27, the needle bearings 28, 28, the ball cam mechanisms 44,44, the thrust bearings 45, 45 and the like. The oil after completion ofthe lubrication is returned from the left and right side casings 13_(L)and 13_(R) through the through-bores 11₁ and 12₁ into the front and rearcenter casings 11 and 12.

Since the oil pump 61 is disposed at the location in which it issandwiched between the left and right electromagnetic clutches C_(L) andC_(R), as described above, the length of the oil passages for supplyingthe oil from the oil pump 61 to the electromagnetic clutches C_(L) andC_(R) can be minimized. Moreover, the oil passages 23₂ and 29₁, 29₁ aredefined to extend through the insides of the clutch drive shaft 23 andthe left and right output shafts 29_(L) and 29_(R) which are connectedin series, a special piping is not required, but also the resistance toflow of the oil is decreased.

If the driven bevel gear 26 and the follower bevel gear 25 are disposedwithin the front and rear center casings 11 and 12, a dead space withtwo ways surrounded by the bevel gears 25 and 26 is created, but theincrease in size of the front and rear center casings 11 and 12 can beprevented by disposing the oil pump 61 utilizing such dead space.Particularly, since the oil pump 61 is comprised of the trochoid pumpwith the inner rotor 67 thereof being fixed to the clutch drive shaft23, the layout of the oil pump 61 in the dead space is facilitated.Moreover, the oil passage 69 connected to the intake port 68 of the oilpump 61 opens directly into the bottoms of the front and rear centercasings 11 and 12 and hence, it is possible to effectively prevent theinclusion of air during inclination of the vehicle V. Further, thefollower bevel gear 25 and the oil pump 61 disposed in the internalspace in the front and rear center casings 11 and 12 exhibit a functionof a baffle plate and hence, it is possible to inhibit waving orturbulence of the oil surface to further effectively prevent theinclusion of air.

Since the casing means of the rear differential D_(R) is divided intothe four portions: the front center casing 11, the rear center casing 12and the left and right side casings 13_(L) and 13_(R), checking andregulation of the meshed states of the driven bevel gear 26 and thefollower bevel gear 25 assembled in the front and rear center casings 11and 12 can be easily carried out by removing the left and right sidecasings 13_(L) and 13_(R) and separating the rear center casing 12 fromthe front center casing 11. Moreover, maintenance of the electromagneticclutches C_(L) and C_(R) assembled within the left and right sidecasings 13_(L) and 13_(R) can be easily carried out by removing only theleft and right side casings 13_(L) and 13_(R) Further, the structure ofa mold used for producing the casing means in a casting process can besimplified, as compared with a case where the casing means is dividedinto only two portions.

Although the preferred embodiment of the present invention has beendescribed in detail, it will be understood that the present invention isnot limited to the above-described embodiment, and various modificationsmay be made without departing from the spirit and scope of the inventiondefined by the claims.

For example, in place of the left and right clutches C_(L) and C_(R)being comprised of the electromagnetic clutches, they may be comprisedof hydraulic clutches. In addition, the present invention is not limitedto a front engine vehicle, and is also applicable to a rear enginevehicle and a midship engine vehicle. The differential having left andright clutches C₁ and C₂ is also not limited to a rear differential butmay also be a front differential.

What is claimed is:
 1. A four wheel drive vehicle comprising:an engine;left and right front wheels and left and right rear wheels; means fortransmitting a driving force from said engine to said front wheels andsaid rear wheels through differentials; an input shaft connecting withone of said differentials for transmitting said driving force thereto;left and right clutches being included in said one differential andbeing interposed between said input shaft and each of said left andright wheels associated with said one differential, said left and rightclutches being capable of selectively coupling either of said left andright wheels to said input shaft with any engagement force; an inputmeans separate from said clutches and operable by the operation of avehicle driver to bring said left and right clutches into engagementwith a maximum engagement force; a wheel speed detecting means fordetecting wheel speeds when said input means is operated; control meansfor controlling the engagement force of said left and right clutches,said control means controlling the engagement force of the clutches toreduce the driving force to said associated wheels in response to anincrease in said wheel speeds and including:releasable cooperatingmembers capable, when engaged, of coupling a selected one of said leftwheel and said right wheel to said input shaft; an electromagnetic coilcarried by one of said clutch members employed for operating theselected wheel; a cam mechanism having a stationary cam fixed withrespect to said electromagnetic coil and a movable cam operative to moveaway from said stationary cam upon the occurrence of relative rotationalmovement between said movable cam and said stationary cam; a clutchpiston operated by said movable cam to interconnect said clutch memberupon relative movement between said cams; and an armature carried by theother of said clutch members employed for operating the selected wheeloperative, upon activation of said coil, to impart relative movementbetween said cams for connecting the clutch member in order to drive theselected wheel.